1. Field of the Invention
This invention relates generally to imaging systems. More specifically, the present invention is directed to systems and methods of increasing the speed of a ray-casting to produce direct volume rendering images in the melt-through interaction mode.
2. Background Discussion
Medical imaging techniques provide doctors and medical technicians with valuable data for patient diagnosis and care. Various imaging techniques include cardiac angiography, peripheral angiography, radiography, computed tomography and positron emission tomography. All of these imaging techniques produce medical images that are studied by medical personnel. A higher quality image leads to more accurate diagnosis.
Radiography is the use of certain spectra of electromagnetic radiation, usually x-rays, to image a human body. Angiography, a particular radiographic method, is the study of blood vessels using x-rays. An angiogram uses a radiopaque substance, or contrast medium, to make the blood vessels visible under x-ray. Angiography is used to detect abnormalities, including narrowing (stenosis) or blockages (occlusions), in the blood vessels throughout the circulatory system and in certain organs.
Cardiac angiography, also known as coronary angiography, is a type of angiographic procedure in which the contrast medium is injected into one of the arteries of the heart, in order to view blood flow through the heart, and to detect obstruction in the coronary arteries, which can lead to a heart attack.
Peripheral angiography, in contrast, is an examination of the peripheral arteries in the body; that is, arteries other than the coronary arteries. The peripheral arteries typically supply blood to the brain, the kidneys, and the legs. Peripheral angiograms are most often performed in order to examine the arteries which supply blood to the head and neck, or the abdomen and legs.
Computed Tomography (CT), originally known as computed axial tomography (CAT or CT scan), is an imaging technique that uses digital geometry processing to generate a three dimensional image of internal features of an object from a series of two-dimensional x-ray images taken around a single axis of rotation. An iodine dye, or other contrast material, may be used to make structures and organs easier to see on the CT picture. The dye may be used to check blood flow, find tumors, and examine other problems.
Positron emission tomography (PET) imaging may also be used. In PET imaging, a short-lived radioactive tracer isotope, which decays by emitting a positron, and which has been chemically incorporated into a metabolically active molecule, is injected into the patient. The radioactive decay of the positrons is measured to generate an image.
When imaging techniques produce images, the images have a dataset of pixels or voxels (described in more detail below) that can be modified to increase the image quality. For example, medical volumetric dataset sizes have been expanding rapidly with the new advanced CT scanners. For example, typical CT machines from Siemens® Medical Solutions can generate a pixel image dataset at a size of 512×512×4096. The capacity to visualize such datasets with high interactivity and high image quality is helpful to medical professionals in diagnosing disease.
Ray-casting is one technique to generate images. However, interactivity is difficult to achieve due to intensive computation and cache-unfriendly memory access. The large number of sampling rays, which grow as a function of O(n2) (where n is image resolution), makes the technique even less efficient.
Volume rendering is one of the most extensively used methods for visualizing volumetric (three-dimensional) data. With three-dimensional datasets becoming more prevalent, volume rendering has come to be widely used in many industrial and research applications. It is of particular importance in clinical scenarios, where radiologists regularly study patient data output from different kinds of scanners (CT, MRI, PET etc.).
There are different volume visualization techniques that can be effectively used to study and understand the volumetric data. The particular technique used depends on the insights desired, and also on the kind of data being visualized. One technique is Direct Volume Rendering (DVR), which is very effective to illustrate the shape of the data features, and can be used with all types of scanner data. In this technique, a color (and opacity) property is assigned to each data value using a Transfer Function. The color value seen at a pixel on the screen is the composition of all the colored data voxels that project to that particular pixel. There are a variety of rendering algorithms for creating a DVR image—some are software-based methods (ray-casting, shearwarp), others use graphics (NVIDIA, ATI) or special hardware (e.g., VolumePro).
Volume visualization environments typically support user interactions in different forms. For example, users usually can perform rotation, translation and scaling (zoom) on the object (volume dataset) being shown, and also cut away parts of the object using clipping planes. One particular interaction mode, which is referred to as melt-through, allows the user to translate and rotate the clipping plane, but does not allow the object to be rotated. The interaction sequence for melt-through proceeds as follows first, the user will position the object as desired using one or a combination of rotation, translation, and scaling. Other interactions like moving the clipping plane are permitted. Next, the user enters the melt through mode and moves the clipping plane forward and backward (translation), or tilt it (rotation) without changing the object orientation and viewing direction. The user can get out of the melt-through mode and rotate the object freely, repeating the steps above.
Melt-through is essentially an interactive operation. To sustain the efficiency of the radiologists, it is very important that the visualization tool provides high frame-rates, high image quality with large image size (often 10242 in size). These requirements pose a number of challenges to the rendering tools.
One challenge is that the high performance of interactive rendering is often achieved by trading the image quality with performance. The final high quality image is rendered only after the user positioned the volume during the interactive mode. High image quality and high performance during the melt-through interaction is a big challenge to the rendering method. In addition, the rendering time of raycaster based method is proportional to the image area size. Enlarging the image from 5122 to 10242 increases the computation of rendering by four times. Finally, with larger and larger datasets generated by medical scanners these days, it is becoming progressively more difficult to maintain high frame-rates during the melt-through operation.
Therefore, it would be an advancement in the state of the art to support a high frame rate with large size and high quality images for the melt-through mode.